Depending on the valve drive architecture, the contact region, referred to below as valve shank contact region, between the contact face of the valve actuating element and the end face on the valve shank is subjected to a high mechanical loading, which can both lead to premature surface wear on one of the two contact partners and also contribute considerably to undesired valve drive friction. In the case of a lever valve drive, therefore, there is a sliding movement of the contact face of the rotationally moved valve actuating element on the end face of the linearly moved valve shank. Here, however, despite high sliding speeds, the formation of a continuous elasto-hydrodynamic lubricating film in the valve shank contact region is hindered on account of the direction reversal of said sliding movement. In connection with high contact pressures, this leads both to a high level of sliding friction, which can generate a significant proportion of the valve drive power loss which is relevant for the fuel consumption of the internal combustion engine, and also to the risk of premature surface wear of the contact partners, in particular in the event of the lubricating film being contaminated with abrasive particles in the lubricating oil.
This consideration also applies to a tappet valve drive whose valve actuating element is moved linearly and generally parallel to the gas-exchange valve. In this case, although no significant sliding speeds in the valve shank contact region result from a lateral migration of the contact face of the valve actuating on the end face on the valve shank, microscopic movements in the valve shank contact region generated by a rotation of the gas exchange valve or of the valve actuating element can lead to similarly undesirable aftereffects with regard to valve drive friction or surface wear.
Conventional measures for wear prevention at the valve shank contact region are firstly inductive hardening of the valve shank end and secondly heat treatment, if appropriate with subsequent nitriding, for surface hardening of the valve actuating element. In the case of very high contact pressures, it is likewise known to weld plates composed of hard metal or hardenable material to the valve shank. Said measures can however have an unsatisfactory effect in particular under the combined loading of the valve shank contact region as a result of high sliding speeds and contact pressures and by contaminated lubricating oil, or in the case of plates which are to be welded on, can lead to undesirably high production costs. Furthermore, no measures are known which, in addition to a permanently sufficient level of wear prevention, bring about a significant reduction of the power loss generated as a result of friction in the valve shank contact region.